This invention relates generally to distributed-optical microfiche readers, and more particularly to a reader of this type that includes an apertured melding mask adapted to effect uniform illumination of the selected image projected on the screen.
In a conventional microfiche storage and retrieval system, individual frames each bearing a reduced scale image of a single document or page of stored intelligence are arranged in a grid formation on a single record sheet or card. This species of microfiche requires a reader in which a selected frame on the microfiche is illuminated and aligned with a single-axis optical system for projection onto a viewing screen.
A single-axis optical system reader has a number of inherent limitations including the need for a minimum of 12 to 17 inches projection distance from the lens to the screen. This materially restricts the potential for reader size and weight reduction. But even if a reader for a conventional microfiche could be made more compact, it is subject to hot-spot overlighting at the center of the screen. This nonuniformity in screen illumination causes eye fatigue, it dictates relatively high power inputs and thereby generates troublesome amounts of heat. Moreover, image resolution and lighting at the corners of the viewing screen are degraded.
The limitations characteristic of conventional microfiche readers have arrested the spread of micrographic technology into many application areas that require portability, maximum user convenience and image readability unaccompanied by eye fatigue.
The serious drawbacks inherent in existing types of microfiche readers have in large measure been overcome by a distributed-optical information storage and retrieval system of the type disclosed in the Waly U.S. Pat. No. 3,704,068 and in the Yevick Pat. Nos. 3,907,420 and 3,864,034, among others, all of which are assigned to the same assignee as the instant application.
In a distributed-optical information storage and retrieval system, the pages of data or intelligence are not recorded in discrete frames as in a conventional microfiche, but are dissected and interlaced thereon to produce a multipage record which is read back by enlarging only that pattern of dispersed characters or bits which together constitute the data of the single recorded page selected for projection.
Instead of a single-axis optical system, use is made of any array of individual lenslets (of the order of 500) which function in parallel, each serving only its own small cell or zone on the viewing screen. Because each individual lenslet acts to project only a tiny piece of the total image onto a respective zone, the projection distance to the screen is reduced to little more than an inch. Thus, the dimensions and weight of the reader can be made to be no greater than that of an average telephone book, thereby affording both full portability and the convenience of either a desk top or lap reader.
In a distributed-optical reader of the type disclosed in the above-identified patents, the microfiche is place in parallel relation to a lens matrix having an array of lenslets each of which is aligned with a respective bit of information on the microfiche to enlarge and project this bit onto a respective screen zone. In some instances, the lens matrix and the microfiche are integrated to form a unitary structure.
Illumination of the microfiche is effected by a cluster of fiber optic light pipes whose inputs are all irradiated by a common light source and whose outputs are in registration with the optical axes of the array of lenslets so that each optical path in the distributed optical system is separately illuminated. Interposed between the lens matrix and the screen and parallel therewith is an apertured mask acting to inhibit the overlap of adjacent information.
In a distributed-optical reader of this type, the light inputs to the several lenslets in the array are of about equal intensity. Consequently, the aggregate light fluxes in the related screen zones are substantially equal. But the intensity of light at any one point within a given zone, say, at zone center, is quite different from the light intensity at a point displaced from center. These zonal differences are governed by certain factors which will be later analyzed.
Hence, while a distributed-optical microfiche reader of the type heretofore known does not suffer from hot-spot overlighting at the center of the screen and all its attendant disadvantages, the screen lighting is not uniform and the illuminated image on the screen has a honeycomb appearance in which each illuminated cell or zone is separately discernible, rather than an image having a uniformly-illuminated seamless form. This honeycomb appearance, which is attributable to the lack of uniform light intensity within each zone forming the total image, though it does not seriously impair the readability of the image, is nevertheless distracting and constitutes a disturbing imperfection. In prior patent to S. Rosin, entitled Distributed-Optical Microfiche Reader, U.S. Pat. No. 4,134,650, issued Jan. 16, 1979, and assigned to the assignee of the present invention, the use of a melding mask to provide more uniform light distribution is disclosed. The melding mask concept disclosed therein, while providing definite improvement over a system without such masks, still provided images with imprecisions and evidence of uneven illumination.